The use of antibodies to block the activity of foreign and/or endogenous polypeptides provides an effective and selective strategy for treating the underlying cause of disease. For example, monoclonal antibodies (MAb), such as the FDA-approved ReoPro (Glaser, V. (1996) “Can ReoPro repolish tarnished monoclonal therapeutics?” Nat. Biotechnol. 14:1216-1217), an anti-platelet MAb from Centocor; Herceptin (Weiner, L. M. (1999) “Monoclonal antibody therapy of cancer” Semin. Oncol. 26:43-51), an anti-Her2/neu MAb from Genentech; and Synagis (Saez-Llorens, X. E., et al. (1998) “Safety and pharmacokinetics of an intramuscular humanized monoclonal antibody to respiratory syncytial virus in premature infants and infants with bronchopulmonary dysplasia” Pediat. Infect. Dis. J. 17:787-791), an anti-respiratory syncytial virus MAb produced by Medimmune, have been used as effective therapeutics.
Standard methods for generating MAbs against candidate protein targets are known by those skilled in the art. Briefly, rodents such as mice or rats are injected with a purified antigen in the presence of adjuvant to generate an immune response (Shield, C. F., et al. (1996) A cost-effective analysis of OKT3 induction therapy in cadaveric kidney transplantation. Am. J. Kidney Dis. 27:855-864). Rodents with positive immune sera are sacrificed and splenocytes are isolated. Isolated splenocytes are fused to melanomas to produce immortalized cell lines that are then screened for antibody production. Positive lines are isolated and characterized for antibody production. The direct use of rodent MAbs as human therapeutic agents were confounded by the fact that human anti-rodent antibody (HARA) responses occurred in a significant number of patients treated with the rodent-derived antibody (Khazaeli, M. B., et al, (1994) Human immune response to monoclonal antibodies. J. Immunother. 15:42-52). In order to circumvent the problem of HARA, the grafting of the complementarity determining regions (CDRs), which are the critical motifs found within the heavy and light chain variable regions of the immunoglobulin (Ig) subunits making up the antigen binding domain, onto a human antibody backbone found these chimeric molecules are able to retain their binding activity to antigen while lacking the HARA response (Emery, S. C., and Harris, W. J. “Strategies for humanizing antibodies” In: ANTIBODY ENGINEERING C. A. K. Borrebaeck (Ed.) Oxford University Press, N.Y. 1995. pp. 159-183). A common problem that exists during the “humanization” of rodent-derived MAbs (referred to hereon as HAb) is the loss of binding affinity due to conformational changes in the 3-dimensional structure of the CDR domain upon grafting onto the human Ig backbone (U.S. Pat. No. 5,530,101 to Queen et al.). To overcome this problem, additional HAb vectors usually need to be engineered by inserting or deleting additional amino acid residues within the framework region and/or within the CDR coding region itself in order to recreate high affinity HAbs (U.S. Pat. No. 5,530,101 to Queen et al.). This process is a very time consuming procedure that involves the use of expensive computer modeling programs to predict changes that may lead to a high affinity HAb. In some instances, the affinity of the HAb is never restored to that of the MAb, rendering them of little therapeutic use.
Another problem that exists in antibody engineering is the generation of stable, high yielding producer cell lines that are required for manufacturing of the molecule for clinical materials. Several strategies have been adopted in standard practice by those skilled in the art to circumvent this problem. One method is the use of Chinese Hamster Ovary (CHO) cells transfected with exogenous Ig fusion genes containing the grafted human light and heavy chains to produce whole antibodies or single chain antibodies, which are a chimeric molecule containing both light and heavy chains that form an antigen-binding polypeptide (Reff, M. E. (1993) High-level production of recombinant immunoglobulins in mammalian cells. Curr. Opin. Biotechnol. 4:573-576). Another method employs the use of human lymphocytes derived from transgenic mice containing a human grafted immune system or transgenic mice containing a human Ig gene repertoire. Yet another method employs the use of monkeys to produce primate MAbs, which have been reported to lack a human anti-monkey response (Neuberger, M., and Gruggermann, M. (1997) Monoclonal antibodies. Mice perform a human repertoire. Nature 386:25-26). In all cases, the generation of a cell line that is capable of generating sufficient amounts of high affinity antibody poses a major limitation for producing sufficient materials for clinical studies. Because of these limitations, the utility of other recombinant systems such as plants are currently being explored as systems that will lead to the stable, high-level production of humanized antibodies (Fiedler, U., and Conrad, U. (1995) High-level production and long-term storage of engineered antibodies in transgenic tobacco seeds. Bio/Technology 13:1090-1093).
Still another aspect of antibody function is the effector mechanisms of the Mab. One of many possible ways to increase effector function of antibodies is via changes in glycosylation. This topic has been recently reviewed by Ruju who summarized the proposed importance of the oligosaccharides found on human IgGs with their degree of effector function (Raju, T S. BioProcess International April 2003. 44-53). According to Wright and Morrison, the microheterogeneity of human IgG oligosaccharides can affect biological functions such as complement dependent cytotoxicty (CDC) and antibody-dependent cytotoxicity (ADCC), binding to various Fc receptors, and binding to C1q protein (Wright A. Morrison S L. TIBTECH 1997, 15 26-32). It is well documented that glycosylation patterns of antibodies can differ depending on the producing cell and the cell culture conditions (Raju, T S. BioProcess International April 2003. 44-53). Such differences can lead to changes in both effector function and pharmacokinetics (Israel E J, Wilsker D F, Hayes K C, Schoenfeld D, Simister N E. Immunology. 1996 December; 89(4):573-578; Newkirk M M, Novick J, Stevenson M M, Fournier Mi, Apostolakos P. Clin. Exp. 1996 November; 106(2):259-64). Differences in effector function may be related to the IgGs ability to bind to the Fcγ receptors (FcγRs) on the effector cells. Shields, et al., have shown that IgG1 with variants in amino acid sequence that have improved binding to FcγR can exhibit up to 100% enhanced ADCC using human effector cells (Shields R L, Namenuk A K, Hong K, Meng Y G, Rae J, Briggs J, Xie D, Lai J, Stadlen A, Li B, Fox J A, Presta L G. J Biol. Chem. 2001 Mar. 2; 276(9):6591-604). While these variants include changes in amino acids not found at the binding interface, both the nature of the sugar component as well as its structural pattern may also contribute to the differences seen. In addition, the presence or absence of fucose in the oligosaccharide component of an IgG1 can improve binding and ADCC (Shields R L, Lai J, Keck R, O'Connell L Y, Hong K, Meng Y G, Weikert S H, Presta L G. J Biol. Chem. 2002 Jul. 26; 277(30):26733-40). An IgG1 that lacked a fucosylated carbohydrate linked to Asn297 exhibited normal receptor binding to the Fcγ receptor. In contrast, binding to the FcγRIIA receptor was improved 50% and accompanied by enhanced ADCC, especially at lower antibody concentrations.
Work by Shinkawa, et al., demonstrated that an antibody to the human IL-5 receptor produced in a rat hybridoma showed more than 50% higher ADCC when compared to the antibody produced in Chinese hamster ovary cells (CHO)(Shinkawa T, Nakamura K, Yaman N, Shoji-Hosaka E, Kanda Y, Sakurada M, Uchida K, Anazawa H, Satoh M, Yamasaki M, Hanai N, Shitara K. J Biol Chem. 2003 Jan. 31; 278(5):3466-73). Monosaccharide composition and oligosaccharide profiling showed that the rat hybridoma-produced IgG1 had a lower content of fucose than the CHO-produced protein. The authors concluded that the lack of fucosylation of an IgG1 has a critical role in enhancement of ADCC activity.
A different approach was taken by Umana, et al., who changed the glycosylation pattern of chCE7, a chimeric IgG1 anti-neuroblastoma antibody (Umana P. Jean-Mairet J, Moudry R, Amstutz H, Bailey J E. Nat Biotechnol. 1999 February; 17(2): 176-80). Using tetracycline, they regulated the activity of a glycosyltransferase enzyme (GnTIII) which bisects oligosaccharides that have been implicated in ADCC activity. The ADCC activity of the parent antibody was barely above background level. Measurement of ADCC activity of the chCE7 produced at different tetracycline levels showed an optimal range of GnTIII expression for maximal chCE7 in vitro ADCC activity. This activity correlated with the level of constant region-associated, bisected complex oligosaccharide. Newly optimized variants exhibited substantial ADCC activity.
A method for generating diverse antibody sequences within the variable domain that results in HAbs and MAbs with high binding affinities to antigens would be useful for the creation of more potent therapeutic and diagnostic reagents respectively. Moreover, the generation of randomly altered nucleotide and polypeptide residues throughout an entire antibody molecule will result in new reagents that are less antigenic and/or have beneficial pharmacokinetic properties. The invention described herein is directed to the use of random genetic mutation throughout an antibody structure in vivo by blocking the endogenous mismatch repair (MMR) activity of a host cell producing immunoglobulins that encode biochemically active antibodies. The invention also relates to methods for repeated in vivo genetic alterations and selection for antibodies with enhanced binding and pharmacokinetic profiles. The methods of the invention may be used to enhance the effector function of the antibodies.
In addition, the ability to develop genetically altered host cells that are capable of secreting increased amounts of antibody will also provide a valuable method for creating cell hosts for product development. The invention described herein is directed to the creation of genetically altered cell hosts with increased antibody production via the blockade of MMR.
The invention facilitates the generation of antibodies with enhanced effector function and the production of cell lines with elevated levels of antibody production. Other advantages of the present invention are described in the examples and figures described herein.